Rust Test Writing Skill Write tests that catch real bugs. Every test must guard a specific invariant -- not just prove the code "works." The "What Could Break?" Framework Before writing any test, answer these four questions: 1. What invariant does this code maintain? (e.g., "deserialized config always has a default profile") 2. What edge case would violate it? (e.g., "empty TOML table, missing key, extra unknown key") 3. What platform difference could surface? (e.g., "path separators, case sensitivity, symlink behavior") 4. What would a future refactor accidentally break? (e.g., "field added…